This proposal investigates the working hypothesis that phytoremediation can be used to degrade airborne PCB congeners from soil and groundwater sources. Plants stimulate the microbial community in the root zone and contribute to microbial degradation of RGBs (rhizodegradation). Higher-chlorinated PCBs are dechlorinated under reducing (anaerobic) conditions;resulting lesser-chlorinated congeners can undergo oxidative mineralization under aerobic conditions. Alternating reducing and oxidizing conditions in the rhizosphere makes the plant-soil system a natural two-stage bioreactor for initial PCB transformation. Lesser-chlorinated PCBs can also be taken-up and transformed inside plan tissues. The specific aims of the project are (1) to test the hypothesis that poplar plants can take up and detoxify lesser-chlorinated PCB congeners by identifying metabolic pathways of PCBs and genes that encode for catabolic enzymes, (2) to test the hypothesis that bacteria in the rhizosphere can reductively dechlorinate higher-chlorinated PCBs and can mineralize resulting lesser-chlorinated congeners under oxidizing conditions;this will be tested using anaerobic and aerobic batch bioreactors with rhizosphere soils contaminated with PCBs, (3) to test the hypothesis that phytoremediation will allow for significant reductions in the airborne transfer of PCBs from waste disposal sites and mitigate exposure to humans and ecosystems;this innovative cleaning up strategy (based on hypotheses 1 and 2) will be tested at the bench scale and by pot-studies in the greenhouse, (4) to test the hypothesis that residues of PCBs in plant tissues are non-toxic or of greatly reduced toxicity to biota by conducting an eco-toxicological evaluation of the phytoremediation process using a battery of toxicity tests, and (5) to test the hypothesis that higher plants play a significant role in the environmental cycling of airborne PCBs by field analyses of PCB accumulation on vegetation. The significance of this project is that it provides an intervention and remedy for contaminated waste sites that will help to break the continuous cycling of PCBs in the atmosphere and the subsequent exposure to humans.